In recent years, in order to prevent the global warming, it has been required to improve the fuel efficiency of the automobiles. For example, an improvement target for the fuel efficiency of the automobiles has been newly set for emission control of carbon dioxide. In order to improve the fuel efficiency of the automobiles, a weight reduction of an automobile body is effective. Thus, for the weight reduction, it has been required to thin the steel sheet for the automobile body. On the other hand, for the safety of the automobile body, it has been required to strengthen the steel sheet for the automobile body.
In addition to the above mentioned requirements of thinning and strengthening the steel sheet, it has been required for the steel sheet for the automobile body which is press-formed to complex shapes to be excellent in surface corrosion resistance and electrophoretic paintability and to be excellent in surface appearance.
In general, in a high tensile strength steel sheet (HTSS), solid solution strengthening elements such as Si (Silicon), Mn (Manganese), P (Phosphorus), or the like are included in the steel for strengthening the steel sheet.
In the galvannealed steel sheet which includes the elements such as Si, Mn, P, or the like as the chemical composition, surface defects such as line defects, stripe defects, or the like may appear after press-forming. Since the surface defects may remain after painting, the surface defects are undesirable for the surface appearance and have been concerns.
In order to suppress the surface defects, it has been mainly proposed to grind a steel piece (slab) before hot-rolling, to grind a hot-rolled steel sheet or a cold-rolled steel sheet before coating, or the like.
For example, as a method for producing a galvannealed steel sheet which has not many patternlike defects on coating surface and in which a steel sheet including Ti (Titanium) and an ultra-low carbon is utilized for base material, Patent Document 1 discloses a method to considerably reduce an amount of scarfing a cast piece or grinding a steel sheet which has been conducted for suppressing the patternlike defects in a manner that element segregation in the cast piece is suppressed by conducting the electromagnetic stirring during continuous casting. As a method for producing a galvannealed steel sheet which is excellent in the surface appearance, coating adhesion, and formability and in which high-Si-based steel sheet or high-P-based steel sheet is utilized for base material, Patent Document 2 discloses a method to grind a surface of a coating steel sheet so as to control surface roughness Ra to 0.3 to 0.6, to immerse it in galvanizing bath, and thereafter, to conduct heating and alloying treatment.
In general, although P is included in the steel to strengthen the steel sheet, P is the element to be readily segregated. Thus, P which is segregated to a slab surface is elongated along a longitudinal direction of the steel sheet by hot-rolling and cold-rolling, and thereby, a concentrated layer of P is formed in a coil surface. Alloying at the concentrated layer of P is delayed during coating, which causes the line defects of the galvannealed steel sheet. For the problem, as a method for producing a galvannealed steel sheet in which a steel sheet including 0.03% or more of P is utilized for base material, Patent Document 3 discloses a method to grind a surface of a steel sheet by a grinding amount depending on a P content in the steel sheet in order to suppress unevenness in the surface of the steel sheet, and thereafter, to conduct alloying treatment by induction heating in an alloying furnace.
In addition, in order to suppress occurrence of chevron stripes on a surface of a pickled steel sheet, Patent Document 4 discloses a method to pickle a hot-rolled steel sheet under ordinary conditions, and thereafter, to further pickle it so as to dissolve a surface layer by 1.0 μm or more.
In order to suppress the linear pattern defects of the galvannealed steel sheet in a case where a steel sheet including Ti, an ultra-low carbon, and 0.03% or more of P is utilized, for example, the prior arts conduct scarfing a surface of a continuous cast piece by 3 mm or more and further grinding a surface of a steel sheet before coating by 5 μm or more. As a result, the prior arts suppress the occurrence of the patternlike defects after coating and obtain surface quality. Even when a steel sheet including Ti, an ultra-low carbon, and a low P is utilized, in the present situation, the prior arts conduct scarfing a surface of a cast piece by 3 mm or more, grinding a surface of a steel sheet after cold-rolling by 2 μm or more using a brush for heavy duty grinding, and scarfing it after pickling by 1 μm or more in order to suppress the chevron stripes. The above-mentioned situation causes a decrease in yield.